Re: Planner choosing NestedLoop, although it is slower...

From: Mario Splivalo
Subject: Re: Planner choosing NestedLoop, although it is slower...
Date: ,
Msg-id: 4E1CEC8C.3000508@megafon.hr
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In response to: Re: Planner choosing NestedLoop, although it is slower...  (Tom Lane)
Responses: Re: Planner choosing NestedLoop, although it is slower...  (Mario Splivalo)
List: pgsql-performance


On 07/13/2011 12:39 AM, Tom Lane wrote:
> Mario Splivalo<>  writes:
>> On 07/12/2011 10:04 PM, Tom Lane wrote:
>>> What you need to look into is why the estimated join size is 9400 rows
>>> when the actual join size is zero.  Are both tables ANALYZEd?  Are you
>>> intentionally selecting rows that have no join partners?
>
>> Yes, both tables have been ANALYZEd. What do you mean, intentilnaly
>> selecting rows taht have no join partners?
>
> I'm wondering why the actual join size is zero.  That seems like a
> rather unexpected case for a query like this.

Yes, seems that planer gets confused by LIMIT. This query:

select * from tubesite_object join tubesite_image on id=object_ptr_id
where site_id = 8 and pub_date < '2011-07-12 13:25:00' order by pub_date
desc ;

Does not choose Nested Loop, and is done instantly (20 ms), and returns
no rows. However, if I add LIMIT at the end, it chooses NestedLoop and
it takes 500ms if I'm alone on the server, and 10+ seconds if there 50+
connections on the server.

    Mario


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From: Lars
Date:
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
From: "Kevin Grittner"
Date:
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database